The world population quadrupled in the 20th century in what the United Nations terms unprecedented growth.

India, China, Pakistan, Nigeria, Bangladesh and Indonesia will account for one half the entire world's population growth over the next 50 years. But U.N. demographers point out that the global migration of people for economic, political or family reasons, also influences a lopsided growth pattern.

About 175 million persons do not live in the nations where they were born. That is only about three percent of the world's population. But, that number has more than doubled in the past 15 years. And, U.N. projections see it continuing to rise as population growth in the developing world outpaces industrialized countries.

Joseph Chamie is Director of the U.N. Population Division and supervisor of the latest International Migration Report. "Virtually all the growth in the next 50 years will be in the developing world, almost all of it - 97-98 percent. 1950 for every person in the developed world there were two in the developing. Today, for every person in the developed world there are four persons. And by 2050, we project for every person in the developed world there will be seven in the developing world," Mr. Chamie said.

Mr. Chamie cites India as an example. Population growth in India over the next half-century, he says, will add as many people as the combined growth in Pakistan, China and Bangladesh.

"Another way to see this. Twenty-five member states of the future European Union: take their growth rate last year, births minus deaths. The entire growth rate of the new European Union, India achieved by January 7 of this year, one week. One week of India's growth equivalent to the European Union, that's how rapid it is," Mr. Chamie said.

The continent of Africa by mid-century is projected to equal three times the EU population.

The U.N. statistics map out the lopsided growth of the world's population. While the number of births is increasing in the developing world, the number of births in the developed world is decreasing and its population is aging. That puts a greater burden on fewer people to support the needs of a developed society.

And, that is where migration has come to play a major role. The U.N. report shows that one out of every ten persons now residing in more industrialized areas of the world is a migrant.

U.N. Population expert Joseph Chamie said, "Let's look at North America. Canada is growing. Why? Migration. Without migration, Canada's population in 2050 would be about the same size as today."

The United States is growing too, also thanks to immigration. Mr. Chamie expects the U.S. population to grow to about 400 million by mid-century.

"They're going to add another 110 million people. Eighty percent of that growth we project will be due to migration - immigrants and their descendants," he said.

Most migrant workers send a bulk of their earnings back home to their families. And that, economists and demographers say, can represent a sizable chunk of the home country's revenues.

Charles Keely is Professor of International Migration at Georgetown University, in Washington. "In Mexico, the issue is six and a half billion dollars going into that economy. It's probably the third largest earner after oil and tourism," Mr. Keely said.

In contrast, Egypt's economy suffered after the 1991 Gulf War forced masses of Egyptian workers to leave their jobs in the Gulf.

The U.N. report also shows that nine percent of the world's migrants are refugees, some 16 million by the end of the year 2000. Two thirds of them are living in developing nations, adding an economic burden to many economies.

Roberta Cohen of the Brookings Institution in Washington is an expert in humanitarian and refugee issues. She adds another 25 million refugees to the U.N. total to account for those refugees who remain within the borders of their own countries. That total represents a sharp increase from the one point two million internally displaced persons registered in 1982.

"We know that the political advantage that motivated many states to accept refugees during the Cold War gave way in the 1990s to a desire to limit their entry. As this inhospitality to asylum seekers grew with increasing numbers of countries finding it too costly, burdensome or destabilizing to admit refugees, the number of those displaced within their own countries began to rise significantly," Mr. Cohen said.

U.N. demographer Chamie has tracked government policies aimed at trying to control the ebb and flow of migrants, including refugees and asylum seekers. He says only about six percent of the world community restricted immigration in the 1970s.

"It's been increasing almost uniformly since 1976. And today, it's 40 percent roughly," he said.

Mr. Chamie adds that the United States and Europe have tightened up their immigration controls even more since the 2001 terrorist attacks against the World Trade Center and the Pentagon.